Optical transmission path having sections which overcompensate for dispersion occurring in the sections

ABSTRACT

An optical communication system which includes a transmission path through which a light is transmitted to a specific point, such as to a receiver. The transmission path includes a plurality of sections so that the light travels through the sections to the specific point. Each section overcompensates for dispersion produced in the respective section for the light so that an amount of dispersion for the light at the specific point is substantially zero.

CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is based on, and claims priority to, Japaneseapplication number Heisei 10-26268, filed on May 8, 1998, in Japan, andwhich is incorporated herein by reference.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an optical communication systemwhich compensates for dispersion. More specifically, the presentinvention relates to an optical communication system having atransmission path with sections which overcompensate for dispersionoccurring in the sections, so that the total dispersion at a pointdownstream of the sections is approximately zero.

[0004] 2. Description of the Related Art

[0005] In ocean transversal long-haul optical communication systemscovering distances of several thousands kilometers, signal transmissionis conducted using optical regenerating repeaters which convert anoptical signal to an electrical signal to perform retiming, reshapingand regenerating.

[0006] However, optical amplifiers which can directly amplify light,without converting the light into an electrical signal, are beinginvestigated for use in optical communication systems. The use of suchoptical amplifiers can greatly reduce the number of parts in a repeater,improve reliability, and drastically reduce cost, as compared to the useof conventional optical regenerating repeaters.

[0007] Moreover, wavelength division multiplexing (WDM) is being used inoptical communication systems in increase transmission capacity. WithWDM, two or more optical signals at different wavelengths aremultiplexed together into a WDM signal. The WDM signal is thentransmitted through a single optical fiber as a transmission line. WDMcan be compared to a conventional optical communication system whereonly one optical signal is transmitted through the optical fiber.

[0008] An optical amplifier, which directly amplifies light withoutconverting the light into an electrical signal, can be used to amplify aWDM signal. In this case, the optical amplifier will simultaneouslyamplify each optical signal in the WDM signal.

[0009] Therefore, an optical communication system which uses WDM incombination with optical amplifiers can provide high capacity, long-hauloptical transmission with a relatively simple, economical structure.Unfortunately, an optical signal transmitted through such an opticalcommunication system can experience a large amount of distortion.

SUMMARY OF THE INVENTION

[0010] Accordingly, it is an object of the present invention to providean optical communication which appropriately compensates for dispersionfor optical signals transmitted through the system.

[0011] Additional objects and advantages of the invention will be setforth in part in the description which follows, and, in part, will beobvious from the description, or may be learned by practice of theinvention.

[0012] The foregoing objects of the present invention are achieved byproviding an optical communication system which includes a transmissionpath through which a light is transmitted to a specific point, such asto a receiver. The transmission path includes a plurality of sections sothat the light travels through the sections to the specific point. Eachsection overcompensates for dispersion produced in the respectivesection for the light so that an amount of dispersion for the light atthe specific point is substantially zero.

[0013] Objects of the present invention are also achieved by providingan optical communication system which includes a transmission pathhaving a plurality of sections so that the light travels through thesections to a specific point. Each section overcompensates fordispersion produced in the respective section for the light to controlthe amount of dispersion for the light at the specific point.

[0014] Objects of the present invention are further achieved byproviding an optical communication system which includes a transmissionpath. Light is transmitted through the transmission path to a specificpoint. The transmission path includes a plurality of sections so thatthe light travels through the sections to the specific point. Eachsection overcompensates for dispersion produced in the respectivesection for the light to reduce the total amount of dispersion for thelight at the specific point.

[0015] Objects of the present invention are also achieved by providing atransmission path including a plurality of sections through which lighttravels to a specific point, wherein the plurality of sections togetherovercompensate for dispersion produced in the sections for the light.The total amount of overcompensation in the sections taken together issubstantially equal to a residual dispersion in the light at thespecific point which would occur if the dispersion for the light in eachsection was approximately zero.

[0016] Further, objects of the present invention are achieved byproviding an optical communication system including a transmission paththrough which a light is transmitted to a specific point, where mdispersion compensators are positioned along the transmission path todivide the transmission path into (m+1) blocks. Each dispersioncompensator overcompensates for dispersion produced in the precedingblock so that the amount of dispersion for the light at the specificpoint is substantially zero.

[0017] In addition, objects of the present invention are achieved byproviding an optical communication system which includes a transmissionpath through which a light is transmitted to a specific point. Adispersion compensator is positioned along the transmission path beforethe specific point and overcompensates for dispersion provided by thetransmission path to the light up to a point along the transmission pathbefore the specific point, so that the amount of dispersion for thelight at the specific point is controlled, reduced, or is substantiallyzero.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] These and other objects and advantages of the invention willbecome apparent and more readily appreciated from the followingdescription of the preferred embodiments, taken in conjunction with theaccompanying drawings of which:

[0019]FIG. 1 is a diagram showing a difference in residual accumulatedwavelength dispersion amount in different wavelengths by influence ofdispersion slope.

[0020]FIG. 2 is a diagram showing an average wavelength of a wavelengthdistribution.

[0021]FIG. 3 is a diagram illustrating a wavelength dispersion map whena wavelength dispersion compensation is applied in an opticalcommunication system.

[0022]FIG. 4 is a diagram showing a section of an optical communicationsystem for compensating for dispersion.

[0023]FIG. 5 is a diagram showing a residual wavelength dispersionamount due to manufacturing error in a transmission path.

[0024]FIG. 6 is a diagram illustrating an optical communication system,according to an embodiment of the present invention.

[0025]FIG. 7 is a diagram illustrating an example of a wavelengthdispersion map for the optical communication system in FIG. 6, accordingto an embodiment of the present invention.

[0026]FIG. 8 is a diagram illustrating a section of a transmission pathof an optical communication system having a wavelength dispersion map asin, for example, FIG. 7, according to an embodiment of the presentinvention.

[0027]FIG. 9 is a diagram illustrating a section of a transmission pathof an optical communication system, according to an additionalembodiment of the present invention.

[0028] FIGS. 10(A), 10(B) and 10(C), and 11(A), 11(B) and 11(C) arediagrams illustrating the use of versatile cables for varying thedispersion compensating amount of an optical communication system,according to an embodiment of the present invention.

[0029]FIG. 12 is a diagram showing multi-core structure of a cable,according to an embodiment of the present invention.

[0030]FIG. 13 is a diagram showing change of fiber length of adispersion compensator, according to an embodiment of the presentinvention.

[0031]FIG. 14 is a diagram showing change of cable length of adispersion compensator, according to an embodiment of the presentinvention.

[0032] FIGS. 15-24 are diagrams showing wavelength dispersion maps,according to embodiments of the present invention.

[0033]FIG. 25 is a diagram showing an optical communication system,according to an embodiment of the present invention.

[0034]FIG. 26 is a diagram illustrating an example of a dispersion mapfor an optical communication system, according to an embodiment of thepresent invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0035] Reference will now be made in detail to the present preferredembodiments of the present invention, examples of which are illustratedin the accompanying drawings, wherein like reference numerals refer tolike elements throughout.

[0036] As indicated above, an optical communication system which usesWDM in combination with optical amplifiers can provide high capacity,long-haul optical transmission with a relatively simple, economicalstructure.

[0037] As an example, a nine-wave multiplex amplifying and repeatingoptical communication system with a transmission rate of 2.5 Gb/s perwave will be explained. In this example, nine signal waves are set inthe range from 1551.0 nm to 1559.0 nm with an interval of 1.0 nm withassignment of channel numbers from the short wavelength side.

[0038] For the transmission path, a 1.5 μm zero dispersion fiber and a1.3 μm zero dispersion fiber are used. The former one is called adispersion shifted fiber (DSF), and the latter one is called adispersion compensating fiber (DCF) because it compensates thedispersion accumulated in the DSF. Dispersion of the DSF should be −2ps/nm/km in average for the wavelength of 1558 nm and that of the DCFshould be +18 ps/nm/km. The repeating interval is set to 70 km.

[0039]FIG. 1 is a diagram showing difference in the residual accumulatedwavelength dispersion amount in each wavelength due to the influence ofdispersion slope. FIG. 1 indicates the condition of the wavelengthdispersion map of three channels, namely, the center wavelength (channel5, in this case), the channel near the shortest wavelength side (channel1, in this case) and the channel near the longest wavelength side(channel 9, in this case) of the signal beam wavelengths.

[0040] When a receiver is positioned at a transmission distance of 3000km, a compensation amount of dispersion to be compensated in thereceiver is different for each wavelength. This difference is due to thedifference in residual dispersion at the receiver in each wavelengthcaused by the dispersion slopes.

[0041]FIG. 2 is a diagram showing that the center wavelength ofdistributed signal beam wavelengths is the average wavelength of thewavelength distribution of the signal beam. In the case of designingwavelength dispersion of the system as a whole, the optimum wavelengthdispersion design of the center wavelength of the distributed signalbeam wavelengths is very important to assure good balance of thetransmission characteristics in all channels.

[0042]FIG. 3 is a diagram showing the wavelength dispersion map in thecenter wavelength (channel 5) of the distributed signal beam wavelengthsin above example. As shown in FIG. 3, as a result of optimum wavelengthdispersion design, compensation of 100% is executed for the accumulatedwavelength dispersion for the transmission path distance of 700 km foreach insertion of DCF having a length of 70 km. Thus, the accumulatedwavelength dispersion becomes zero for each insertion of the DCF.Thereafter, above structure is repeated along the transmission path.

[0043] In FIG. 3, after a transmission of 3000 km, the signal beam isdispersion-compensated (post-compensation) using a DCF in the receiver.More specifically, a dispersion amount indicated by a dotted line inFIG. 3 is compensated by a dispersion compensator, such as a DCF,provided in the receiver. The channels other than the center wavelengthchannel 5 are also dispersion-compensated by adjusting the length of DCFfor each channel (signal beam wavelength) at the receiver because theresidual dispersion accumulated in the transmission path by theinfluence of the dispersion slope is different depending on thecharacteristics in FIG. 1.

[0044]FIG. 4 is a diagram showing a section of an optical communicationsystem providing the wavelength dispersion map of FIG. 3. Referring nowto FIG. 4, a transmission distance of 700 km is realized by utilizingten optical amplifiers/repeaters OA1 through OA10 spaced apart from eachother in intervals of 70 km. DSF is used as the transmission fiberbetween the optical amplifiers/repeaters. The transmission path betweenoptical amplifier/repeater OA10 and an optical amplifier/repeater OA11is formed of a DCF. In this case, as illustrated in FIG. 4, dispersioncompensation is conducted so that dispersion becomes zero at the 770 kmpoint.

[0045] When a dispersion compensation technique as in FIGS. 3 and 4 isused, and when the transmission path becomes longer, the amount ofdispersion compensating fiber at the receiver increases. As a result,the receiver will likely increase in size.

[0046] In addition, the dispersion amount to be compensated in thereceiver must be adjusted to compensate for wavelength dispersiongenerated by manufacturing error of wavelength dispersion value in theDSF and DCF used in the transmission path.

[0047] Moreover, the residual dispersion is different in each wavelengthdue to the influence of the dispersion slope. Therefore, the dispersioncompensation amount compensated in the receiver is different for eachwavelength. If residual wavelength dispersion at the center wavelengthis large, a large difference is generated between the channels havinggood and bad transmission characteristics. As a result, balance of thetotal system is lost and the size of the receiver becomes large.

[0048] For example, transmission for the distance of 10,000 km (oceantransversal distance) will be explained. Since the center wavelength ofthe distributed signal beam wavelengths is thought to be important,attention will be paid to this wavelength. Here, for example, it isassumed that wavelength dispersion in the DSF is set to −1.8 ps/nm/km,wavelength dispersion in the DCF is set to +18 ps/nm/km, the DCF isinserted in every other ten repeating points for every repeatingdistance of 70 km, and the accumulated wavelength dispersion of everyDCF is compensated by 100%. As a calculation result using these typicalvalues without any consideration of manufacturing error, a DCF of about1260 ps/nm is required at the receiver. This is a relatively large DCF.

[0049] Moreover, manufacturing error generated when the fiber isactually manufactured must also be considered.

[0050]FIG. 5 is a diagram showing the residual wavelength dispersionamount of a transmission path due to manufacturing error of a DSF and aDCF forming the transmission path. More specifically, FIG. 5 shows theresidual wavelength dispersion amount when the manufacturing error ofwavelength dispersion of a 1.5 μm zero dispersion DSF is ±0.2 ps/nm/kmand the manufacturing error of wavelength dispersion of a DCF is ±0.5ps/nm/km.

[0051] In accordance with the above, the rate of use of DSF and DCF inthe transmission path is almost identical to an inverse number ratio ofthe wavelength dispersion and is about 10:1. In this case, the residualdispersion reaches ±2275 ps/nm, where manufacturing error of the DSF is±1820 ps/nm and that of the DCF is ±455 ps/nm. In order to compensatefor this residual dispersion, a DCF about ±120 km is necessary as theextra fiber. This distance corresponds to four sections.

[0052] As explained above, the DCF of 3535 ps/nm in maximum (1260+2275ps/nm) is required to compensate for the residual wavelength dispersionat the receiver, by the technique described in FIGS. 1-4. In order torealize this dispersion compensation amount, a DCF of about 200 km isnecessary. When loss of the fiber is assumed as 0.2 dB/km, the lossgenerated in this DCF is about 40 dB.

[0053] Here, two units of optical amplifiers having a gain of about 20dB are required to compensate for this loss. Therefore, the structureand installation size of the receiver become large. Moreover, ifresidual dispersion is large, deterioration of transmissioncharacteristic may be generated.

[0054] Therefore, as can be seen from above, when a signal istransmitted by WDM as shown in FIG. 1, dispersion compensation must beconducted for all channels in the receiver. Additional compensation forerror of dispersion compensation in the receiver is also necessary toextend further the transmission distance. Accordingly, wavelengthdispersion compensation at the receiver is necessary to preventdeterioration of the transmission characteristics.

[0055] Moreover, as can be seen from above, the technique disclosed inFIGS. 3-4 typically requires a large amount of dispersion compensationat the receiver. It would be desirable to reduce or eliminate thisamount of dispersion compensation, to thereby reduce the size, cost andcomplexity of the receiver.

[0056] In contrast to the technique disclosed in FIGS. 3-4, and as willbe described in more detail below, according to embodiments of thepresent invention, a DCF having a dispersion compensating amount largerthan the accumulated wavelength dispersion amount is periodicallyinserted to approximate the residual dispersion to be compensated at thereceiver to zero. As a result, in a single wave communication system,dispersion compensation in the receiver may be reduced or eliminated.

[0057] In addition, as will be discussed in more detail below, accordingto embodiments of the present invention, with WDM communication, theresidual dispersion to be compensated by the center wavelength at thereceiver is approximated to zero. The other wavelengths may bedistributed around zero to reduce the dispersion compensation amount inthe receiver. As a result, the size of the receiver can be reducedwithout deterioration of the transmission characteristics.

[0058] Moreover, as will be discussed in more detail below, according toembodiments of the present invention, since the dispersion compensatingamount of the DCF inserted to the transmission path may be varied andfluctuation of residual dispersion amount by the manufacturing error canbe reduced, the dispersion compensating amount in the receiver can bereduced.

[0059]FIG. 6 is a diagram illustrating an optical communication system,according to an embodiment of the present invention. In FIG. 6, DCFs areinserted into the transmission path to divide the transmission path intoblocks. Residual dispersion is not set to zero in blocks for eachinsertion of the DCF. Instead, the final residual dispersion becomessubstantially zero for the center wavelength in the distribution of asignal beam wavelength in a WDM communication system, or for thecommunication wavelength in a single wavelength communication system.

[0060] More specifically, referring now to FIG. 6, an n-wave signal beamis multiplexed in a transmitter 100 and is then demultiplexed in areceiver 110. The signal beam travels from transmitter 100 to receiver110 through a transmission path 120. In transmission path 120, adispersion compensating fiber DCF for dispersion compensation isintroduced, for example, in every other several sections of DSF.

[0061] In a WDM communication system, for the center wavelength ofdistribution in the signal beam wavelength, the dispersion compensationamount of the DCF used for dispersion compensation is not set to zero atthe output of the DCF. Instead, the use of DSFs and DCFs in transmissionpath 120 causes the final residual dispersion amount at receiver 110 tobe approximately zero.

[0062] Particularly, when the dispersion compensating interval is equal,compensation amount of the DCF inserted into transmission path 120exceeds the dispersion amount accumulated during transmission by DSFs upthat point along transmission path 120. When the number of sections ofDCF is defined as (m) and the number of blocks of DSF required forcompensation (corresponding to several sections of DSF) as (m+1),compensation of about ((m+1)/m)×100% is required.

[0063]FIG. 7 is a diagram illustrating an example of a wavelengthdispersion map for the optical communication system in FIG. 6, accordingto an embodiment of the present invention. In this example, wavelengthdispersion in the DSF is set to −1.8 ps/nm/km, wavelength dispersion inthe DCF is set to +18 ps/nm/km for the center wavelength of distributionin the signal beam wavelength, the transmission distance is 10,000 km,the transmission distance of one block of DSF (typically correspondingto several sections of DSF) is set to 700 km, and the transmissiondistance of one section of DCF is set to 76 km. One block of DSF plusone section of DCF can be considered to be one section of transmissionpath 120.

[0064] As shown in FIG. 7, in each section of transmission path 120which includes a DCF, residual wavelength dispersion is not set to zero,but it finally becomes zero at the receiver. Namely, when the dispersioncompensating interval is equal, if the number of sections of DCF is setto (m) and the number of blocks of DSF required for compensation is setto (m+1), compensation amount of the DCF may be set to about((m+1)/m)×100%.

[0065]FIG. 8 is a diagram illustrating a section of a transmission pathhaving a wavelength dispersion map as in, for example, FIG. 7, accordingto an embodiment of the present invention. Here, it is assumed that theDCFs divide the transmission path into blocks, and the section of thetransmission path illustrated in FIG. 8 includes one block of DSF and anassociated section of DCF.

[0066] Referring now to FIG. 8, the dispersion value of the section ofthe transmission path exceeds 0. A positive dispersion value can beobtained by setting each transmission path DSF from the transmitter tooptical amplifier/repeater OA1, and between optical amplifier/repeatersOA1 to OA10, to, for example, 70 km, respectively, and by inserting aDCF of, for example, 76 km, between optical amplifier/repeaters OA10 andOA11.

[0067] In FIGS. 7 and 8, the transmission path DSF has minus dispersionvalue, while the DCF has plus dispersion value.

[0068]FIG. 9 is a diagram illustrating a section of a transmission path,where the signal is transmitted for 10,000 km, the transmission path DSFhas plus dispersion value and the DCF has minus dispersion value,according to an embodiment of the present invention. As illustrated inFIG. 9, the dispersion value does not become zero in each section of thetransmission path, but the residual dispersion amount is finally set tozero at the position where the receiver is installed.

[0069] In FIGS. 8 and 9, for a WDM optical communication system, thedispersion value is not set to zero in each section of the transmissionpath where the DCF is inserted for the center wavelength of distributionin the signal beam wavelength. Instead, the final residual dispersion isset to substantially zero. However, for WDM, when dispersioncompensation is conducted for the particular wavelength forcommunication, it is also possible to conduct dispersion compensation sothat the dispersion value does not become zero for the particularwavelength in the section of the transmission path having the particularwavelength. As an example of the particular wavelength, the wavelengthof a channel to transmit high bit rate signal and the wavelength ofchannel required to have a low error rate can be considered. Thetransmission quality can be improved by arranging the wavelengths ofthese channels as the center wavelengths of total wavelengths.

[0070]FIGS. 8 and 9 illustrate all the DSFs as providing positivedispersion, or all the DSFs as providing negative dispersion. However,the present invention is not intended to be limited to this. Instead,for example, the transmission path can include a mixture of DSFsproviding positive and negative dispersion. Similarly, the DCFs are notlimited to all providing positive dispersion, or all providing negativedispersion. Instead, the transmission path can have a mixture of DCFsproviding positive or negative dispersion.

[0071] According to embodiments of the present invention, it may bepossible to vary the dispersion compensating amount of DCFs inserted tothe transmission path, adjust the dispersion amount and reducefluctuation of the residual dispersion amount.

[0072] For example, the manufacturing error of wavelength dispersion ofa DSF might vary by several percent. As a result, for example, if themanufacturing error of the DSF deviates to a negative accumulated value,the manufacturing error can be compensated by increasing the dispersioncompensating value of a DCF inserted to the transmission path. If themanufacturing error of the DSF deviates to a positive accumulated value,the manufacturing error can be compensated by reducing dispersion amountof a DCF inserted to the transmission path.

[0073] To compensate for manufacturing error in the dispersion amount ofthe DSF, the dispersion compensating amount of a DCF inserted to thetransmission path can be varied by, for example, (a) using versatilecables; (b) using cables having an increased number of fiber cores whichcan be selected; (c) changing the fiber length of the DCF; and (d) usingDCFs having versatile wavelength dispersion values.

[0074] FIGS. 10(A), 10(B), 10(C) and 11(A), 11(B) and 11(C) are diagramsillustrating the use of versatile cables for varying the dispersioncompensating amount, according to an embodiment of the presentinvention. A cable corresponding to the required dispersion compensatingvalue can be selected from cables providing different amounts ofdispersion compensation.

[0075]FIG. 12 is a diagram illustrating a multi-core cable for varyingthe dispersion compensating amount, according to an embodiment of thepresent invention. As illustrated in FIG. 12, the multi-core cableincludes versatile fibers in different dispersion compensating amountswithin a single cable. The fiber of the optimum dispersion compensatingamount is selected at the time of cable connection. The structure shownin FIG. 12 is economically preferable in comparison with a case where asingle fiber is prepared for a single cable, and a case where aplurality of cables having different dispersion values are prepared,because cable is generally more expensive than fiber.

[0076] The dispersion compensation amount can also be varied by changingfiber length or cable length without changing the installation positionof optical amplifiers/repeaters within a certain section.

[0077] For example, FIGS. 13 and 14 are diagrams showing a DCF of adispersion compensating section of a transmission path being physicallyformed in a longer length than the cable, so that the dispersioncompensation amount can be varied, according to an embodiment of thepresent invention.

[0078] In FIG. 13, the DCF length can be set longer than the cablelength by changing the radius of curvature when the DCF is wound to thetensile force line within the cable. The DCF length can be adjusted bychanging a radius of curvature.

[0079] In FIG. 14, the cable laying section, namely opticalamplifier/repeater interval, is set in the same distance as that of theother transmission section. The radius of curvature for meandering thecable when it is laid is selected to a value different from that of theDSF. Thereby, distance can substantially be extended or reduced tochange the dispersion compensating amount.

[0080] Versatile wavelength dispersion compensating values of fibers canalso be used to vary the wavelength dispersion compensating amount. Inthis case, versatile dispersion compensating fibers are prepared indifferent wavelength dispersion values in a certain section. Manufacturecan be done easily by changing a ratio of the lengths of DSF and DCFforming the dispersion compensating section.

[0081] To realize a compact structure and installation at the receiverwithout deterioration of transmission characteristic, it may beeffective to use a combination of the above-described techniques forvarying the dispersion compensating amount based on the dispersion errorfor each dispersion compensating section of the transmission path.

[0082] In an example as explained above, the dispersion compensatingamount can be adjusted and fluctuation of the residual wavelengthdispersion due to the manufacturing error of fiber can be compensated bypreparing versatile DCF in different dispersion amounts.

[0083] As an example, assume that wavelength dispersion in the DSF isset to −1.8 ps/nm/km, wavelength dispersion in the DCF is set to +18ps/nm/km, and DCF is inserted in every other ten repeaters of therepeating distance of 70 km for the transmission distance of 10,000 km.Moreover, assume that the manufacturing error of wavelength dispersionof the DSF is set to ±0.2 ps/nm/km, and the manufacturing error ofwavelength dispersion of the DCF is set to ±0.5 ps/nm/km. In this case,since fluctuation of ±2275 ps/nm is generated in the residual dispersiondue to the manufacturing error, fluctuation of about ±120 km isgenerated as the dispersion compensating fiber length. To control suchfluctuation, for example, as described above, cables having differentrates of dispersion compensating fiber can be used.

[0084] First, three kinds of fiber in the rate of dispersioncompensating fiber of 0%, 50% and 100% are assumed, considering themerits in the manufacturing process and economical aspect. If residualwavelength dispersion amount of the system as a whole is deviated tonegative, the rate of the DCF is changed to 100% from 50% and if it isdeviated to positive, the rate of the DCF is changed to 0% from 50%, byarranging the section where the rate of DCF is 50% when themanufacturing error is not included.

[0085] Residual wavelength dispersion can be adjusted in the wavelengthdispersion value step of a half section of the repeating section.Namely, when the repeating section is 70 km, the residual wavelengthdispersion can be adjusted in the step of about 700 ps/nm.

[0086] Moreover, since fluctuation of about ±120 km exists as the fiberlength, about three sections (3×35 km) are required in this case tocompensate for such fluctuation.

[0087] An example where the section having the rate of the dispersioncompensating fiber of 50% is introduced up to three sections will beexplained below.

[0088]FIG. 15 is a diagram showing the wavelength dispersion map notincluding manufacturing error, according to an embodiment of the presentinvention. Here, it is assumed that the optimum wavelength dispersiondesign is conducted when manufacturing error is not included. In FIG.15, the repeating section length is 70 km.

[0089]FIG. 16 is a diagram showing the wavelength dispersion mapincluding the manufacturing error of wavelength dispersion of DSF of+0.2 ps/nm/km and manufacturing error of wavelength dispersion ofdispersion compensating fiber +0.5 ps/nm/km, according to an embodimentof the present invention. The repeating section length is 70 km.

[0090] On the other hand, FIG. 17 is a diagram showing the wavelengthdispersion map when the wavelength dispersion of the system as a wholeis adjusted, because dispersion is deviated to positive, by changing therate of dispersion compensating fiber to 0% from 50%, according to anembodiment of the present invention. In FIG. 17, the wavelengthdispersion map includes a manufacturing error of wavelength dispersionof DSF of +0.2 ps/nm/km and a manufacturing error of wavelengthdispersion of dispersion compensating fiber of +0.5 ps/nm/km. In FIG.17, the repeating section length is 70 km.

[0091] Moreover, FIG. 18 is a diagram showing the wavelength dispersionmap including a manufacturing error of wavelength dispersion of DSF of−0.2 ps/nm/km and a manufacturing error of wavelength dispersion ofdispersion compensating fiber of −0.5 ps/nm/km, according to anembodiment of the present invention. In FIG. 18, the repeating sectionlength is 70 km.

[0092] Meanwhile, FIG. 19 is a diagram showing the wavelength dispersionmap when the wavelength dispersion of the system as a whole is adjusted,because dispersion is deviated to negative, by changing the rate ofdispersion compensating fiber to 100% from 50%, according to anembodiment of the present invention. In FIG. 19, the wavelengthdispersion map includes a manufacturing error of wavelength dispersionof DSF of −0.2 ps/nm/km and a manufacturing error of wavelengthdispersion of dispersion compensating fiber of −0.5 ps/nm/km. Therepeating section length is 70 km.

[0093] Therefore, as explained above, fluctuation by the manufacturingerror can be controlled.

[0094] Next, the case where the repeating interval is set to 50 km willbe explained. Since the fluctuation of about ±120 km exists within thefiber length, about four repeating sections (4×25 km) will be requiredfor compensating such fluctuation.

[0095] The case where the section having the rate of DCF of 50% is used,up to four sections will be explained. Since the repeating section is 50km, residual wavelength dispersion amount of the system can be adjustedin the step of about 500 ps/nm.

[0096]FIG. 20 is a diagram showing the wavelength dispersion map notincluding manufacturing error, according to an embodiment of the presentinvention. Here it is assumed that optimum wavelength dispersion designis conducted when the manufacturing error is not included. The repeatingsection length is 50 km.

[0097]FIG. 23 is a diagram showing the wavelength dispersion mapincluding a manufacturing error of wavelength dispersion of DSF of +0.2ps/nm/km and a manufacturing error of wavelength dispersion ofdispersion compensating fiber of +0.5 ps/nm/km, according to anembodiment of the present invention. The repeating section length is 50km.

[0098] On the other hand, FIG. 24 is a diagram showing the wavelengthdispersion map when wavelength dispersion of system as a whole isadjusted by changing the rate of dispersion compensating fiber to 0%from 50% because dispersion is deviated to positive, according to anembodiment of the present invention. In FIG. 24, the wavelengthdispersion map includes a manufacturing error of wavelength dispersionof DSF of +0.2 ps/nm/km and a manufacturing error of wavelengthdispersion of dispersion compensating fiber of +0.5 ps/nm/km. Therepeating section length is 50 km.

[0099] Moreover, FIG. 21 is a diagram showing the wavelength dispersionmap including a manufacturing error of wavelength dispersion of DSF of−0.2 ps/nm/km and a manufacturing error of wavelength dispersion ofdispersion compensating fiber of −0.5 ps/nm/km, according to anembodiment of the present invention. The repeating section length is 50km.

[0100]FIG. 22 is a diagram showing the wavelength dispersion map whenthe wavelength dispersion of system as a whole is adjusted by changingthe rate of dispersion compensating fiber to 100% from 0% becausedispersion is deviated to positive, according to an embodiment of thepresent invention. In FIG. 22, the wavelength dispersion map includes amanufacturing error of wavelength dispersion of DSF of −0.2 ps/nm/km anda manufacturing error of wavelength dispersion of dispersioncompensating fiber of −0.5 ps/nm/km. The repeating section length is 50km.

[0101] As explained above, fluctuation by the manufacturing error can becontrolled. Moreover, when it is required to set the adjusting step ofresidual wavelength dispersion smaller value, it can be realized byfurther increasing kinds of the rates of the dispersion compensatingfiber.

[0102]FIG. 25 is a diagram showing an optical communication systemhaving a wavelength dispersion adjusting section for compensating themanufacturing error of wavelength dispersion of the system as a whole,according to an embodiment of the present invention. Referring now toFIG. 25, a wavelength dispersion adjusting section 200 is provided alongtransmission path 120.

[0103] In wavelength dispersion adjusting section 200, various of theabove-described techniques can be used to adjust wavelength dispersion.For example, in wavelength dispersion adjusting section 200, an optimumfiber can be selected at both terminal stations. Therefore, the finalwavelength dispersion can be equalized, for example, by selecting anadequate fiber from those having versatile dispersion values preparedfrom the fiber and cable formed as described above.

[0104] In FIG. 25, characteristics of the fibers laid in the course oflaying the fibers respectively from the transmitter and receiver areinvestigated. Wavelength dispersion error of the system as a whole isadjusted using any one or combination of the techniques disclosed hereinconsidering the section for connecting the cable laid from thetransmitter and the cable laid from the receiver as wavelengthdispersion adjusting section 200. The dispersion amount is adjusted sothat the transmission characteristics do not deteriorate, by adjustingthe dispersion amount to make substantially zero the wavelengthdispersion at the receiver position.

[0105] According to embodiments of the present invention, when a signalis transmitted in WDM mode, it is no longer required to conductdispersion compensation for an average wavelength of a wavelengthdistribution in the receiver and accumulated dispersion for all channelscan be reduced. Moreover, since adjustment is conducted using the centerwavelength in WDM mode, fluctuation of dispersion of the otherwavelengths becomes almost zero, and total amount of dispersion in thedispersion compensator required in the receiver can be reduced.Accordingly, an optical amplifier for compensating loss by thedispersion compensator can be eliminated in the receiver.

[0106] In addition, conventionally, it was required to compensate forthe error of dispersion compensation in the receiver to expand thetransmission distance. However, according to embodiments of the presentinvention, the dispersion compensating amount in the receiver can bereduced or eliminated because an error is also compensated by thedispersion compensator provided in the transmission path.

[0107] According to the above embodiments of the present invention, anoptical amplifier provided for compensating loss by the dispersioncompensator can be eliminated. Moreover, in various embodiments of thepresent invention, a dispersion compensator in a receiver can beeliminated or reduced in size.

[0108] According to the above embodiments of the present invention, anoptical communication system includes a transmission path through whicha light is transmitted to a specific point, such as to a receiver. Thetransmission path includes a plurality of sections so that the lighttravels through the sections to the specific point. For example, inFIGS. 8 and 9, each block of optical amplifiers optically connectedtogether, along with the DCF, forms a respective section of thetransmission path. As illustrated in FIGS. 8 and 9, each sectionovercompensates for dispersion produced in the respective section forthe light so that an amount of dispersion for the light at the specificpoint is controlled, reduced, or made to be substantially zero.

[0109] Moreover, according to the above embodiments of the presentinvention, an optical communication system includes a transmission pathhaving a plurality of sections through which light travels to a specificpoint. The plurality of sections together overcompensate for dispersionproduced in the sections for the light. The total amount ofovercompensation in the sections taken together is substantially equalto a residual dispersion in the light at the specific point which wouldoccur if the dispersion for the light in each section was approximatelyzero. Therefore, generally, a specific amount of dispersion compensationis required to compensate for dispersion occurring in portions of thetransmission path other than the section providing dispersioncompensation. This specific amount of dispersion compensation isessentially “distributed” to the sections. For example, in FIGS. 8 and9, dispersion compensation is required to compensate for dispersionoccurring between optical amplifier/repeaters OA10 and OA11. Accordingto embodiments of the present invention, this amount of dispersion isessentially distributed to the various sections, so that the totalamount of overcompensation provided by the sections substantially equalsthat required to compensate for dispersion occurring between opticalamplifiers OA10 and OA11.

[0110] According to embodiments of the present invention, m dispersioncompensators are positioned along the transmission path to divide thetransmission path into (m+1) blocks. Each dispersion compensatorovercompensates for dispersion produced in the preceding block so thatthe amount of dispersion for the light at the specific point issubstantially zero. For example, see FIGS. 8 and 9.

[0111] Moreover, according to embodiments of the present invention, anoptical communication system includes a transmission path through whicha light is transmitted to a specific point. A dispersion compensator ispositioned along the transmission path before the specific point. Thedispersion compensator overcompensates for dispersion provided by thetransmission path to the light up to a point along the transmission pathbefore the specific point, so that the amount of dispersion for thelight at the specific point is controlled, reduced, or is substantiallyzero. According to the above embodiments of the present invention,various blocks or sections of a transmission path include opticalamplifiers/repeaters. For example, FIGS. 8 and 9 illustrate one block asincluding ten optical amplifiers/repeaters. However, a block or sectionis not intended to be limited to having any specific number of opticalamplifiers/repeaters. Instead, the number of opticalamplifiers/repeaters used in a specific configuration will depend, forexample, on the design specifications of the system.

[0112] According to the above embodiments of the present invention, aDCF is typically positioned in a section of a transmission line afterthe last optical amplifier/repeater in the section. For example, inFIGS. 8 and 9, a DCF is positioned after the last opticalamplifier/repeater OA 10. However, the present invention is not intendedto be limited to this positioning of the DCF. Instead, a DCF can bepositioned anywhere along the section. Moreover, a DCF is not intendedto be limited to being a single DCF positioned in the section. Instead,many different DCFs positioned at different locations in a section ofthe transmission path can together be considered as being a DCF ordispersion compensator. Thus, for example, in FIGS. 8 and 9, eachsection can have, for example, two DCFs positioned somewhere along thesection, where the two DCFs, taken together, provide the required amountof dispersion compensation.

[0113] The present invention will also be operable as far as the totaldispersion at a specific point is controlled to become substantiallyzero. This is shown in FIG. 26 as another embodiment of the presentinvention. In FIG. 26, the dispersion in at least one section isovercompensated, and the dispersions in other sections areundercompensated.

[0114] According to various embodiments of the present invention,sections of a transmission path overcompensate for dispersion producedin the respective sections so that an amount of dispersion for lighttravelling through the sections to a the specific point (such as thelocation of a receiver) is substantially zero. However, the presentinvention is not intended to be limited to controlling the dispersion atthe specific point to be “substantially zero”. Instead, the sections canbe seen as simply controlling or reducing the amount of dispersion atthe specific point. Thus, the dispersion is not limited to beingsubstantially zero at the specific point.

[0115] According to the above embodiments of the present invention, adispersion compensator is used to provide dispersion overcompensation.The dispersion compensator can be, for example, a variable dispersioncompensator which is controllable to vary the amount ofovercompensation. For example, the various configurations in FIGS. 10-14can be considered to be variable dispersion compensators, since theamount of dispersion provided by these configuration can be changed.Typically, with these configurations, the amount of dispersion providedby the dispersion compensator is set during installation. However, avariable dispersion compensator can be used in which the amount ofdispersion compensation is changeable after installation and/or afteroperation of the system. Moreover, the present invention is not intendedto be limited to dispersion compensators formed of dispersioncompensating fibers, and other types of dispersion compensators can beused.

[0116] Although a few preferred embodiments of the present inventionhave been shown and described, it would be appreciated by those skilledin the art that changes may be made in these embodiments withoutdeparting from the principles and spirit of the invention, the scope ofwhich is defined in the claims and their equivalents.

What is claimed is:
 1. An optical communication system comprising: atransmission path through which a light is transmitted to a specificpoint, the transmission path including a plurality of sections so thatthe light travels through the sections to the specific point, eachsection overcompensating for dispersion produced in the respectivesection for the light so that an amount of dispersion for the light atthe specific point is substantially zero.
 2. An optical communicationsystem as in claim 1, further comprising: a transmitter; and a receiverat the specific point, the transmitter transmitting the light to thereceiver through the transmission path.
 3. An optical communicationsystem as in claim 1, wherein the light includes at least onewavelength.
 4. An optical communication system as in claim 1, whereinthe light is a wavelength division multiplexed signal.
 5. An opticalcommunication system as in claim 1, wherein the light includes aplurality of different wavelengths including a center wavelength, andeach section overcompensates for the dispersion produced in therespective section for light at the center wavelength so that the amountof dispersion for the light at the center wavelength at the specificpoint is substantially zero.
 6. An optical communication system as inclaim 1, further comprising: a plurality of optical amplifiers arrangedalong the transmission path in equal intervals.
 7. An opticalcommunication system as in claim 1, wherein each section comprises: aplurality of optical amplifiers arranged along the transmission path;and a dispersion compensator overcompensating for dispersion produced inthe section.
 8. An optical communication system as in claim 7, whereinthe optical amplifiers in each section are arranged along thetransmission path in equal intervals.
 9. An optical communication systemas in claim 1, wherein each section comprises: a plurality of opticalamplifiers arranged along the transmission path in equal intervals froma first optical amplifier to a last optical amplifier; and a dispersioncompensator overcompensating for dispersion produced in the section. 10.An optical communication system as in claim 6, wherein the amount ofovercompensation of each section is changeable without changing thepositions of the optical amplifiers.
 11. An optical communication systemas in claim 7, wherein, for each section, the amount of overcompensationis changeable without changing the positions of the optical amplifiersin the section.
 12. An optical communication system as in claim 1,wherein each section includes a cable providing dispersion to the lightas the light travels through the cable, to thereby cause the section toovercompensate for dispersion produced in the section.
 13. An opticalcommunication system as in claim 1, wherein each section includes acable selectable from the group consisting of a cable having adispersion generating fiber through which the light travels, a cablehaving a dispersion compensating fiber through which the light travels,and a cable having a dispersion generating fiber and a dispersioncompensating fiber connected together so that the light travels throughboth the dispersion generating fiber and the dispersion compensatingfiber, to vary the amount of dispersion compensation provided by thesection.
 14. An optical communication system as in claim 12, wherein thecable includes a plurality of fibers providing different amounts ofdispersion and which are individually selectable so that the lighttravels through the selected fiber, the cable thereby allowing theamount of overcompensation provided by the section to be selected. 15.An optical communication system as in claim 14, wherein a fiber of thecable is selectable during installation of the system.
 16. An opticalcommunication system as in claim 12, wherein the cable is a multi-corecable having individually selectable cores so that the light travelsthrough the selected core, the cable thereby allowing the amount ofovercompensation provided by the section to be selected.
 17. An opticalcommunication system as in claim 1, wherein when the transmission pathprovides a positive dispersion to the light, each sectionovercompensates for the dispersion produced in the respective section byadding a negative dispersion to the light, and when the transmissionpath provides a negative dispersion to the light, each sectionovercompensates for the dispersion produced in the respective section byadding a positive dispersion to the light.
 18. An optical communicationsystem as in claim 1, wherein, in addition to travelling through theplurality of sections, the light travels through an additional portionof the transmission path to reach the specific point.
 19. An opticalcommunication system as in claim 1, wherein, in addition to travellingthrough the plurality of sections, the light travels through anadditional portion of the transmission path to reach the specific point,the total amount of overcompensation provided by the plurality ofsections being substantially equal to the amount of dispersion producedin said additional portion.
 20. An optical communication system as inclaim 7, wherein the dispersion compensator is a variable dispersioncompensator which is controllable to vary the amount ofovercompensation.
 21. An optical communication system comprising: atransmission path through which a light is transmitted to a specificpoint, the transmission path including a plurality of sections so thatthe light travels through the sections to the specific point, eachsection overcompensating for dispersion produced in the respectivesection for the light to control the amount of dispersion for the lightat the specific point.
 22. An optical communication system as in claim21, further comprising: a transmitter; and a receiver at the specificpoint, the transmitter transmitting the light to the receiver throughthe transmission path.
 23. An optical communication system as in claim21, wherein the light includes a plurality of different wavelengthsincluding a center wavelength, and each section overcompensates for thedispersion produced in the respective section for light at the centerwavelength so that the amount of dispersion for the light at the centerwavelength at the specific point is substantially zero.
 24. An opticalcommunication system as in claim 21, wherein each section comprises: aplurality of optical amplifiers arranged along the transmission path;and a dispersion compensator overcompensating for dispersion produced inthe section.
 25. An optical communication system as in claim 21, whereineach section comprises: a plurality of optical amplifiers arranged alongthe transmission path in equal intervals from a first optical amplifierto a last optical amplifier; and a dispersion compensatorovercompensating for dispersion produced in the section.
 26. An opticalcommunication system as in claim 25, wherein the dispersion compensatoris a variable dispersion compensator which is controllable to vary theamount of overcompensation.
 27. An optical communication system as inclaim 21, wherein when the transmission path provides a positivedispersion to the light, each section overcompensates for the dispersionproduced in the respective section by adding a negative dispersion tothe light, and when the transmission path provides a negative dispersionto the light, each section overcompensates for the dispersion producedin the respective section by adding a positive dispersion to the light.28. An optical communication system as in claim 21, wherein, in additionto travelling through the plurality of sections, the light travelsthrough an additional portion of the transmission path to reach thespecific point, the total amount of overcompensation provided by theplurality of sections being substantially equal to the amount ofdispersion produced in said additional portion.
 29. An opticalcommunication system comprising: a transmission path through which alight is transmitted to a specific point, the transmission pathincluding a plurality of sections so that the light travels through thesections to the specific point, the plurality of sections togetherovercompensating for dispersion produced in the sections for the lightto reduce the total amount of dispersion for the light at the specificpoint.
 30. An optical communication system as in claim 29, furthercomprising: a transmitter; and a receiver at the specific point, thetransmitter transmitting the light to the receiver through thetransmission path.
 31. An optical communication system as in claim 29,wherein the light includes a plurality of different wavelengthsincluding a center wavelength, and each section overcompensates for thedispersion produced in the respective section for light at the centerwavelength so that the total amount of dispersion for the light at thecenter wavelength at the specific point is substantially zero.
 32. Anoptical communication system as in claim 29, wherein each sectioncomprises: a plurality of optical amplifiers arranged along thetransmission path; and a dispersion compensator overcompensating fordispersion produced in the section.
 33. An optical communication systemas in claim 29, wherein each section comprises: a plurality of opticalamplifiers arranged along the transmission path in equal intervals froma first optical amplifier to a last optical amplifier; and a dispersioncompensator overcompensating for dispersion produced in the section. 34.An optical communication system as in claim 33, wherein the dispersioncompensator is a variable dispersion compensator which is controllableto vary the amount of overcompensation.
 35. An optical communicationsystem as in claim 29, wherein when the transmission path provides apositive dispersion to the light, each section overcompensates for thedispersion produced in the respective section by adding a negativedispersion to the light, and when the transmission path provides anegative dispersion to the light, each section overcompensates for thedispersion produced in the respective section by adding a positivedispersion to the light.
 36. An optical communication system as in claim29, wherein, in addition to travelling through the plurality ofsections, the light travels through an additional portion of thetransmission path to reach the specific point, the total amount ofovercompensation provided by the plurality of sections beingsubstantially equal to the amount of dispersion produced in saidadditional portion.
 37. An optical communication system comprising: atransmission path through which a light is transmitted to a specificpoint, the transmission path including a plurality of sections throughwhich the light travels to the specific point, the plurality of sectionstogether overcompensating for dispersion produced in the sections forthe light, a total amount of overcompensation in the sections takentogether being substantially equal to a residual dispersion in the lightat the specific point which would occur if the dispersion for the lightin each section was approximately zero.
 38. An optical communicationsystem as in claim 37, further comprising: a transmitter; and a receiverat the specific point, the transmitter transmitting the light to thereceiver through the transmission path.
 39. An optical communicationsystem as in claim 37, wherein the light includes a plurality ofdifferent wavelengths including a center wavelength, and the pluralityof sections together overcompensate for the dispersion produced in thesections for light at the center wavelength so that the amount ofdispersion for the light at the center wavelength at the specific pointis substantially zero.
 40. An optical communication system as in claim37, wherein each section comprises: a plurality of optical amplifiersarranged along the transmission path; and a dispersion compensatorovercompensating for dispersion produced in the section.
 41. An opticalcommunication system as in claim 37, wherein each section comprises: aplurality of optical amplifiers arranged along the transmission path inequal intervals from a first optical amplifier to a last opticalamplifier; and a dispersion compensator overcompensating for dispersionproduced in the section.
 42. An optical communication system as in claim40, wherein the dispersion compensator is a variable dispersioncompensator which is controllable to vary the amount ofovercompensation.
 43. An optical communication system as in claim 37,wherein when the transmission path provides a positive dispersion to thelight, the plurality of sections together overcompensate for dispersionby adding a negative dispersion to the light, and when the transmissionpath provides a negative dispersion to the light, the plurality ofsections together overcompensate for dispersion by adding a positivedispersion to the light.
 44. An optical communication system as in claim37, wherein, in addition to travelling through the plurality ofsections, the light travels through an additional portion of thetransmission path to reach the specific point, the total amount ofovercompensation provided by the plurality of sections beingsubstantially equal to the amount of dispersion produced in saidadditional portion.
 45. An optical communication system comprising: atransmission path through which a light is transmitted to a specificpoint; m dispersion compensators positioned along the transmission pathto divide the transmission path into (m+1) blocks, each dispersioncompensator overcompensating for dispersion produced in the precedingblock so that the amount of dispersion for the light at the specificpoint is substantially zero.
 46. An optical communication system as inclaim 45, wherein the dispersion compensators are equally spaced alongthe transmission path, and each dispersion compensator provides adispersion amount of approximately ((m+1)/m)·100%.
 47. An opticalcommunication system as in claim 45, wherein each dispersion compensatorprovides a dispersion amount of approximately ((m+1)/m)·100%.
 48. Anoptical communication system as in claim 45, further comprising: atransmitter; and a receiver at the specific point, the transmittertransmitting the light to the receiver through the transmission path.49. An optical communication system as in claim 45, wherein the lightincludes a plurality of different wavelengths including a centerwavelength, and each dispersion compensator overcompensates for thedispersion produced in the proceeding block for light at the centerwavelength so that the amount of dispersion for the light at the centerwavelength at the specific point is substantially zero.
 50. An opticalcommunication system as in claim 45, wherein each block comprises: aplurality of optical amplifiers arranged along the transmission path.51. An optical communication system as in claim 45, wherein each blockcomprises: a plurality of optical amplifiers arranged along thetransmission path in equal intervals.
 52. An optical communicationsystem as in claim 45, wherein at least one of the dispersioncompensators is a variable dispersion compensator which is controllableto vary the amount of overcompensation provided by the dispersioncompensation.
 53. An optical communication system as in claim 45,wherein when the transmission path provides a positive dispersion to thelight, the dispersion compensators overcompensate for dispersion byadding a negative dispersion to the light, and when the transmissionpath provides a negative dispersion to the light, the dispersioncompensators overcompensate for dispersion by adding a positivedispersion to the light.
 54. An optical communication system as in claim45, wherein, in addition to travelling through the blocks, the lighttravels through an additional portion of the transmission path to reachthe specific point, the total amount of overcompensation provided by thedispersion compensators being substantially equal to the amount ofdispersion produced in said additional portion.
 55. An opticalcommunication system comprising: a transmission path through which alight is transmitted to a specific point; a dispersion compensatorpositioned along the transmission path before the specific point andovercompensating for dispersion provided by the transmission path to thelight up to a point along the transmission path before the specificpoint, so that the amount of dispersion for the light at the specificpoint is substantially zero.
 56. An optical communication system as inclaim 55, further comprising: a transmitter; and a receiver at thespecific point, the transmitter transmitting the light to the receiverthrough the transmission path.
 57. An optical communication system as inclaim 55, wherein the light includes a plurality of differentwavelengths including a center wavelength, and the dispersioncompensator overcompensates for dispersion so that the amount ofdispersion for the light at the center wavelength at the specific pointis substantially zero.
 58. An optical communication system as in claim55, further comprising: a plurality of optical amplifiers equally spacedalong the transmission path.
 59. An optical communication system as inclaim 55, wherein the dispersion compensator is a variable dispersioncompensator which is controllable to vary the amount ofovercompensation.
 60. An optical communication system as in claim 55,wherein when the transmission path provides a positive dispersion to thelight, and the dispersion compensator overcompensates for dispersion byadding a negative dispersion to the light, and when the transmissionpath provides a negative dispersion to the light, the dispersioncompensator overcompensates for dispersion by adding a positivedispersion to the light.
 61. An optical communication system comprising:a transmission path through which a light is transmitted to a specificpoint; a dispersion compensator positioned along the transmission pathbefore the specific point and overcompensating for dispersion providedby the transmission path to the light up to a point along thetransmission path before the specific point, to control the amount ofdispersion for the light at the specific point.
 62. An opticalcommunication system as in claim 61, further comprising: a transmitter;and a receiver at the specific point, the transmitter transmitting thelight to the receiver through the transmission path.
 63. An opticalcommunication system as in claim 61, wherein the light includes aplurality of different wavelengths including a center wavelength, andthe dispersion compensator overcompensates for dispersion so that theamount of dispersion for the light at the center wavelength at thespecific point is substantially zero.
 64. An optical communicationsystem as in claim 61, further comprising: a plurality of opticalamplifiers equally spaced along the transmission path.
 65. An opticalcommunication system as in claim 61, wherein the dispersion compensatoris a variable dispersion compensator which is controllable to vary theamount of overcompensation.
 66. An optical communication system as inclaim 61, wherein when the transmission path provides a positivedispersion to the light, and the dispersion compensator overcompensatesfor dispersion by adding a negative dispersion to the light, and whenthe transmission path provides a negative dispersion to the light, thedispersion compensator overcompensates for dispersion by adding apositive dispersion to the light.
 67. A method comprising: providing atransmission path through which a light is transmitted to a specificpoint; and overcompensating for dispersion provided by the transmissionpath to the light up to a point along the transmission path before thespecific point, to control the amount of dispersion for the light at thespecific point.
 68. A method as in claim 67, wherein saidovercompensating causes the amount of dispersion for the light at thespecific point to be substantially zero.
 69. A method as in claim 67,wherein the light includes a plurality of different wavelengthsincluding a center wavelength, and said overcompensating overcompensatesfor dispersion so that the amount of dispersion for the light at thecenter wavelength at the specific point is substantially zero.
 70. Amethod as in claim 67, further comprising: providing a plurality ofoptical amplifiers equally spaced along the transmission path.
 71. Amethod as in claim 67, wherein when the transmission path provides apositive dispersion to the light, said overcompensating overcompensatesfor dispersion by adding a negative dispersion to the light, and whenthe transmission path provides a negative dispersion to the light, saidovercompensating overcompensates for dispersion by adding a positivedispersion to the light.
 72. A method comprising: providing atransmission path through which a light is transmitted to a specificpoint, the transmission path including a plurality of sections so thatthe light travels through the sections to the specific point; and ineach section, overcompensating for dispersion produced in the respectivesection for the light so that an amount of dispersion for the light atthe specific point is substantially zero.
 73. A method as in claim 72,wherein the light includes a plurality of different wavelengthsincluding a center wavelength, and in each section, saidovercompensating overcompensates for the dispersion produced in therespective section for light at the center wavelength so that the amountof dispersion for the light at the center wavelength at the specificpoint is substantially zero.
 74. An apparatus comprising: a transmissionpath through which a light is transmitted to a specific point; and meansfor overcompensating for dispersion provided by the transmission path tothe light up to a point along the transmission path before the specificpoint, to control the amount of dispersion for the light at the specificpoint.
 75. An optical communication system comprising: a transmissionpath including a section which overcompensates for dispersion occurringin the section so that total dispersion for light travelling through thesection is approximately zero at a point downstream of the section. 76.An optical communication system as in claim 75, wherein the transmissionpath includes a plurality of sections which together overcompensate fordispersion occurring in the plurality of section so that totaldispersion for light travelling through the plurality of sections isapproximately zero at a point downstream of the plurality of sections.77. An optical communication system comprising: a transmission paththrough which a light is transmitted to a specific point, thetransmission path including a plurality of sections so that the lighttravels through the sections to the specific point, at least one sectionovercompensating for dispersion produced in the respective section forthe light so that an amount of dispersion for the light at the specificpoint is substantially zero.